intel_idle: Eliminate __setup_broadcast_timer()

The __setup_broadcast_timer() static function is only called in one
place and "true" is passed to it as the argument in there, so
effectively it is a wrapper arround tick_broadcast_enable().

To simplify the code, call tick_broadcast_enable() directly instead
of __setup_broadcast_timer() and drop the latter.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>